Nondestructive Inspection (NDI) overview
Nondestructive inspection methods have revolutionized the way in which components for various industries are tested and qualified. With increasing technological advancements, introduction to multiple different testing techniques which are non-invasive have led to the improvement in quality and efficiency, for aerospace, automotive and medical device components. Industry leaders often choose to outsource nondestructive testing projects to inspection labs for quick and accurate testing results.
What is NDI inspection?
NDI inspection stands for Nondestructive Testing Inspection, which is synonymous with Nondestructive Testing (NDT) and Nondestructive Evaluation (NDE). The choice of terms is the preference of the user. All three terms are used to describe testing, inspection or evaluation of an object without destroying the part being inspected. Nondestructive inspection describes a set of different analysis techniques which are utilized to identify part properties for quality control purposes.
The retrieved data from nondestructive inspection techniques can be used to identify part reliability, detect and evaluate flaws and ensure part consistency.
Types of NDI methods
There are six primary methods of nondestructive inspection including magnetic particle testing (MT), liquid penetrant testing (PT), radiographic testing (RT), ultrasonic testing (UT), electromagnetic testing (ET) and visual testing (VT). Below, all nondestructive inspection methods are briefly explained:
Surface Examination Methods
Visual Testing (VT) – users check and evaluate the surface integrity by viewing the part surface with the naked eye, or with camera systems.
Magnetic Particle Testing (MT) – This technique is effective in detecting surface flaws. In order for this technique to be utilized, the part being tested must be of ferromagnetic material, where a magnetic field is induced and distorted where are discontinuities are present.
Liquid Penetrant Testing (PT) – The object being tested is coated with florescent dye. This liquid is consumed by defective areas such as porosity and cracks, and all excessive liquid is removed.
Electromagnetic Testing (ET) – In order for this technique to be utilized, the part being tested must consist of electrically conductive materials. A magnetic field is induced which generates electric eddy current. Any resistance (caused by flaws) is captured on a recording device called voltmeter.
Volumetric Examination Methods
Ultrasonic Testing (UT) – This technique for nondestructive inspection utilizes frequency of sound waves to identify flaws. The varying acoustic nature of the materials being tested is used to classify failures.
Radiographic Testing (RT) – A source of radiation is utilized to inspect external and internal flaws of a part being inspected. The resulting image is evaluated by the varying densities, and the location and orientation of failures can be determined. Radiographic testing can be conducted with results displayed on radiographic film, or captured by advanced detectors and evaluated digitally.
Condition Monitoring Methods
Infrared Testing (IR) – The thermal profile of a part being tested is displayed in graph form. The variations and discontinuities can be identified by attaining work temperature assessment data.
Vibration Analysis (VA) – Determines condition of a machine by producing vibration noise and documenting the frequency.
Integrity Examination Methods
Leak Testing (LT) – This method of NDI testing is regularly used to locate leaks in pressure absorbent parts, such as pipes. Multiple different types of leak testing techniques exist including soap-bubble test, liquid and gas penetrant techniques or electronic listening devices to name a few.
Acoustic Emission Testing (AE) – Emission is acoustic energy which is released when a part is under stress. Special receivers can detect the intensity, which can provide insight to the location of a defect.
Special NDT Methods
Guided Wave Testing (GW) – As detailed within ASNT, Guided wave testing on piping uses controlled excitation of one or more ultrasonic waveforms that travel along the length of the pipe, reflecting from changes in the pipe stiffness or cross sectional area.
Laser Testing Methods (LM) – Laser Testing includes three distinct methods: Holography, Shearography and Profilometry; using lasers, these techniques are utilized to perform the inspection and detect defects and flaws.
Magnetic Flux Leakage (MFL) – As detailed within ASNT, Magnetic Flux Leakage detects anomalies in normal flux patterns created by discontinuities in ferrous material saturated by a magnetic field.
Neutron Radiographic Testing (NR) – Neutron radiography is similar in process and application with radiographic testing, with the only main difference being the use of an intense beam of low energy neutrons as a penetrating medium rather than the gamma- or x-radiation used in conventional radiography.
Uses for NDI inspection
The uses of NDI testing vary, subject to the industry and specific project needs. Although there are many different approaches on how to utilize and evaluate nondestructive testing data, there are dominant uses common across all industries:
- Evaluate part components for consistency and reliability
- Test and inspect part components
- Failure analysis tool
- Quality control management tool
- Means of inspection, without destroying subject
- Identify failures, continuities or flaws
- Measurement and dimensional analysis tool
- Residual life assessment tool
- Detect conditions by wear, fatigue, corrosion, pressures or stress
- Qualification of parts – approval process
- Research and development
- Ensure product repeatability
- Evaluate equipment condition and reliability
- Accident prevention
Benefits of NDI
Since nondestructive inspection methods do not apply any external forces or pressure to the object being scanned, the part is available for use after the inspection process is completed. This reduces the cost and time of manufacturing additional test parts. The testing results can exponentially increase the quality of the final component, as well as provide insightful data on the internal and external structure, failures and dimensional analysis without causing any damage to the part in its entirety.
Applications for NDI
NDI inspection can be applied in any stage of a parts manufacturing cycle. With access to Nondestructive inspection techniques during pre-production all the way to production and failure analysis stages, a part can be analyzed and inspected without destruction. Applications of NDI testing include:
- Weld verification
- Research & development
- Design and prototyping
- Pre-production qualification
- Production failures or approval process
- Failure analysis
- Archiving data
- Reverse engineering
- Part to CAD comparison for consistency
- Dimensional analysis
- First Article Inspection reports
- Quality control
- PPAP requirements
- AS9102 requirements
Certification for NDI
A form of certification for Nondestructive inspection is authorized by the American Society of Nondestructive Testing (ASNT). For this particular certification, in order to utilize and operate nondestructive testing equipment, users must have a pre-determined number of training hours under supervision and must complete a written test requirement. A certification can be awarded in two distinct ways:
Employer based certification: An employer highlights the number of hours necessary for training, experience under supervision and the written examination requirements. The written examinations are usually based on the ASNT industry practices (SNT-TC-1A).
Personal Central Certification: A certification authority which is approved and recognized by employers and industry is utilized to attain a NDT certification. Industry standards must be followed and similarly, a number of training hours, experience under supervision and a written exam is required.
Levels of certification for NDI
There are three distinct levels of certification under nondestructive inspection which describe the roles and responsibilities of individuals within each level.
Level 1: Restricted to perform only certain calibrations or tests under supervision by a higher level individual. They are able to report results, and are expected to follow instructions regarding procedure.
Level 2: Open to set up and calibrate inspection equipment, perform inspection according to standards and codes. These individuals are able to provide instruction to level 1 technicians, as well as report, interpret and evaluate results. Have knowledge of standards and codes.
Level 3: Specialized, experienced and qualified engineers or technicians. These technicians are able to direct NDT labs, and have extensive knowledge about testing, service and manufacturing processes as well as codes and standards.